Fibres and material comprising same

ABSTRACT

A method for producing a cement material with reduced development of self-induced cracking, the method comprising: 
     adding to a concrete, mortar or cement mix to which water has been added, an amount of 0.05 to 1% by weight, based on the cementitious materials, of synthetic fiber bundles comprising 10-10,000 filaments per bundle, the filaments consisting essentially of a polyolefin, polyolefin derivative, a polyester or a mixture of the foregoing and having a length of 1-30 mm, a mean transverse dimension of 5-30 um and an aspect ratio of 100 to 1000, the filaments in each bundle being held together by a wetting agent, the wetting agent proving the individual filaments with a surface tension which allows them to become homogeneously dispersed in a concrete, mortar or paste mix with conventional mixing in conventional concrete mixing equipment, 
     mixing the resultant mix for a period of at least 20 seconds to obtain a concrete, mortar or cement mix in which the individual filaments are homogeneously dispersed, and 
     casting the concrete, mortar or paste mix in a configuration.

This application is a continuation of application Ser. No. 07/996,511filed on Dec. 21, 1992, which is a continuation application of Ser. No.07/689,078 filed on Jun. 24, 1991, both abandoned.

BACKGROUND OF THE INVENTION

The use of various types of fibres in the production of concrete, toprovide additional tensile strength and reinforce against impact damageand crack propagation, has been known and practiced for a long time. Itis also known that while conventional reinforcement and coarser fibrescan reduce the larger visible cracking which tends to occur in concrete,only very fine fibres are really effective in combating the developmentof smaller cracks. However, the fibres which are generally used inconcrete, for example synthetic fibres of materials such aspolypropylene, are relatively coarse, due to the fact that it isdifficult to achieve a satisfactory dispersion in concrete of very finefibres, and particularly fibres with high aspect ratios, usingconventional mixing procedures and equipment. In fact, the uniformdispersion of even relatively coarse fibres in concrete can also bedifficult.

It is common for such fibres to be produced as an integral fibrillatedtape, and to rely on extended mixing to break down the fibrillation andto disperse the individual filaments, which are still relatively coarse,within the concrete. This system may not always be reliable, and thefibrillated tape is not always broken down into the desired individualfilaments, especially since the degree of extended mixing required is inpractice frequently not achieved. Even when effectively separated, thefibres may still be too coarse to achieve maximum effectiveness forcrack inhibition, particularly against micro-cracking.

Concrete is prone to self-induced cracking and, as it is a brittlematerial, these cracks propagate readily under relatively low stresses.Concrete fails in tension by progressive crack development rather thanthe more usual failure mode of engineering materials.

It is generally assumed that the discrepancy between concrete's actualand theoretical strength can be explained by the presence of flaws(Neville, A.M., Properties of Concrete, 1981). Thus, concrete does notcrack because it is weak in tension, but rather it is weak in tensionbecause it already contains cracks. These cracks and flaws vary in size,so that scale is very important when dealing with fracture mechanics, inthat the actual strength of the whole is a matter of statisticalprobability which is dependent upon the crack distribution within thematerial. The effective strength of the concrete can therefore beincreased, and failure, i.e. the development of large-scale cracks orfractures, can be prevented, by inhibiting the development andpropagation of cracks.

Self-induced, non-structural cracks occur in large masses of ready-mixedconcrete due to small cracks which form early, and these aresubsequently propagated by stresses induced by changes in the dimensionsof such relatively large structures. Pavement concrete units aretypically about 3 m by 10 m by 200 mm thick; small cracks in suchconcrete can readily propagate, producing a weak link which results insubsequent fracture. This clearly visible cracking is often the onlyform of cracking which is perceived as being of importance, but it is adirect result of much smaller and probably essentially invisible earliercrack development.

EP-A-0 235 577 discloses agglomerates of fibres having improveddispersability in viscous organic or inorganic matrices, e.g.cement-based matrices, comprising acrylic staple fibres, each fibrehaving a diameter of less than 50 μm and length of more than 3 mm, thefibres being bonded to each other by a cohesion-conferring agent whichis dissolved in, swells or melts in the matrix to be reinforced. Thecohesion-conferring agent, e.g. polyvinyl alcohol, is applied in anamount of 1-30% by weight of the fibre. The fibres preferably have ahigh elastic modulus.

EP-A-0 225 404 discloses a method of manufacturing a fibre-reinforcedmoulded cement body, which comprises dispersing strands comprising aplurality of fibres into an unhardened cement material and thereafterhardening the material, at least some of the strands being impregnatedwith a binder so that the fibres of the strands are weakly bound to oneanother and so that, when the strands are dispersed in the cementmaterial, the fibres are released from one another. The binder, e.g. anepoxy resin, is used in an amount such that the ratio of the strands tothe binder is from 5:5 to 9:1 by volume.

Previous applications of fibre in concrete have been directed towardsconventional reinforcement, where sufficient fibres of high elasticmodulus are used to bear the tensile stresses. Although this is possiblein high cement content materials, this approach may not work effectivelywith more conventional concretes, even with steel fibres havingexcellent mechanical characteristics. This can be attributed to thefollowing:

a. The volume of fibre required can be too great to be accommodated inthe mortar phase of concrete.

b. The benefit of the fibres may be achieved after the matrix hasfailed, and can thus in such cases be described as simply beingprogressive failure rather than usable strength.

c. The cost and difficulties in use do not always justify theapplication.

d. The three-dimensional orientation of the fibres in premix use and theuse of the fibres throughout the material often comprise an inefficientuse of reinforcement.

It has become increasingly evident that the most important commercialcontribution of the fibres is to improve the characteristics of theconcrete itself, rather than to act independently as a reinforcement.

Reinforcement is, however, easy to measure, and although the otherbenefits, i.e. the strengthening of the concrete itself, may berecognized as being of significance, the difficulty in measuring andquantifying them has been a factor in inhibiting this application offibre in concrete.

BRIEF DISCLOSURE OF THE INVENTION

It has now been found that it is possible to employ very smallquantities of very fine synthetic fibres, for example of polypropylene,to improve the characteristics and performance of concrete and mortars,in particular to prevent the development of cracks induced bydimensional changes occurring within the concrete, and to achieve thiscrack control at the important micro-level before the cracks develop tobecome visually evident. The fibres thus serve to improve the intrinsicstrength of the concrete, and in particular to prevent self-inducedcracks from developing at the micro-level as well as to prevent theirpropagation, instead of merely providing a separate independentreinforcement.

The fibres are incorporated into the concrete or mortar in the form offibre bundles which, as will be explained below, allow the desiredsubstantially homogeneous distribution of fine fibres in the material tobe achieved. One aspect of the present invention relates therefore tosynthetic fibre bundles designed for use in concrete, mortar or cement,the bundles comprising 10-10,000 filaments per bundle, the filamentsconsisting essentially of a polyolefin such as polypropylene orpolyethylene, a polyolefin derivative, a polyester or a mixture of theforegoing and having a length of 1 to 30 mm, a mean transverse dimensionof 5 to 30 μm and an aspect ratio of 100 to 1000, the filaments in eachbundle being held together by a wetting agent, the wetting agentproviding the individual filaments with a surface tension which allowsthem to become substantially homogeneously dispersed in a concrete,mortar or paste with conventional mixing in conventional concrete mixingequipment.

Another aspect of the invention relates to cement-based materialscomprising a small amount of the above fibres. This aspect thus relatesto a cement-based concrete, mortar or paste having substantiallyhomogeneously distributed therein synthetic fibres comprising apolyolefin such as polypropylene or polyethylene, a polyolefinderivative, a polyester or a mixture of the foregoing and having alength of 1 to 30 mm, a mean transverse dimension of 5 to 30 μm and anaspect ratio of 100 to 1000, the surface of the fibres comprising awetting agent, the fibres being present in an amount of less than about1% by weight of the cementitious materials of the concrete, mortar orpaste. The expression "material of the invention", as used in thefollowing, refers to such materials.

In a further aspect, the invention is related to a method of producingthe above cement-based material, the method comprising:

adding to a concrete, mortar or cement mix to which water has been addedless than 1% by weight, based on the cementitious materials, ofsynthetic fibre bundles comprising 10-10,000 filaments per bundle, thefilaments comprising a polyolefin such as polypropylene or polyethylene,a polyolefin derivative, a polyester or a mixture of the foregoing andhaving a length of 1 to 30 mm, a mean transverse dimension of 5 to 30 μmand an aspect ratio of 100 to 1000, the filaments in each bundle beingheld together by a wetting agent, the wetting agent providing theindividual filaments with a surface tension which allows them to becomesubstantially homogeneously dispersed in the mix with conventionalmixing in conventional concrete mixing equipment,

mixing the resulting mix for a period of at least about 20 seconds toobtain a concrete, mortar or paste mix in which the individual filamentsare substantially homogeneously distributed, and

casting the concrete, mortar or paste mix in a desired configuration,optionally with incorporation, during the casting, of additional bodiessuch as reinforcement.

The invention also relates to a method of producing synthetic fibrebundles for use in concrete, mortar or cement, the bundles comprising10-10,000 filaments per bundle, the filaments consisting essentially ofa polyolefin such as polypropylene or polyethylene, a polyolefinderivative, a polyester or a mixture of the foregoing, the methodcomprising:

melting the fibre raw material(s) to obtain a melt,

spinning the melt into spun bundles of filaments,

stretching the bundles of filaments,

drying and fixing the bundles of filaments, such that the stretchedfilaments after fixing have a mean transverse dimension of 5 to 30 μm,

treating the bundles of filaments with a wetting agent so as to hold thefilaments of each bundle together and to provide the filaments with asurface tension which allows them to become substantially homogeneouslydispersed in a concrete, mortar or paste with conventional mixing inconventional concrete mixing equipment, and

cutting the bundles of filaments to a length of 1 to 30 mm, such thatthe individual filaments have an aspect ratio of 100 to 1000.

It has been found that because they are initially present in the form offibre bundles, the very fine fibres described above (in the followingreferred to as "fibres of the invention") are capable of beingeffectively dispersed in all types of concrete, mortar or cement usingall types of existing conventional mixers, including the rotating drumof a ready-mixed concrete truck. The fibres of the invention can, sincethey are capable of becoming well dispersed, even at very low additionrates give many important advantages to the characteristics andperformance of concrete and other cement-based materials. Among theseadvantages are:

The inhibition of cracks resulting from plastic shrinkage, plasticsettlement, early thermal shrinkage, chemical shrinkage and carbonation.

The inhibition of cracks resulting from drying shrinkage, alternatingcyclical stresses due to imposed loads, reversible moisture movementsand thermal changes.

The possible elimination of the need for steel mesh to control the worstproblems caused by self-induced cracking.

An improvement in durability, due to crack prevention and reduced waterabsorption, and increased intrinsic strength as a result.

A reduction in frost damage, due to reduced permeability and anincreased resistance to spalling as a result of increased concretestrength and integrity and increased resistance to crack propagation.

Increased resistance to impact and abrasive damage.

Greater cohesion of the wet concrete and the imparting of a thixotropicrheology, resulting in:

more homogeneous and consistent concrete, with more uniform and reliablecharacteristics,

easier pumping, placing and finishing, and the prevention ofsedimentation and excessive bleeding,

a reduced tendency for the formation of plastic settlement cracks, as aresult of the reduction in sedimentation, and

benefits when placing the concrete on slopes, as the material has lesstendency to continue movement, which otherwise results in increasedtendency to cracking.

Better resistance to fire damage, since the fine synthetic fibres meltat high temperatures, thus allowing the superheated steam generatedwithin the concrete a large number of capillaries by which to escape.

Better resistance to deterioration by corrosive chemicals, due toreduced penetration of such chemicals into the concrete.

A more consistent and homogeneous concrete by increasing mixerefficiency and preventing particle size segregation and subsequentsedimentation.

The cohesiveness provided by the fibres of the invention serves toimprove the finishing of the concrete. Texturing of the surface, toproduce a non-slip finish, is improved, and the achieved effect is notreduced by subsequent settlement, due to the thixotropic characteristicprovided by the fibres.

The fibres do not impair the surface finish of the concrete and are,themselves, effectively invisible in the concrete to the naked eye. Thethixotropic effect may also be of interest in enabling new and moreinteresting aesthetic finishes and effects to be achieved, includingdecorative in situ and pre-cast applications.

It is increasingly common to employ higher cement contents in concreteand other cement-based materials, in order to increase their durability.This leads, however, to a greater tendency to self-induced cracking,and, because these materials are relatively brittle, to greater crackpropagation. As mentioned above, the fibres of the invention are capableof being effectively dispersed in all types of concrete or mortar. Thefibres, due to their ability to prevent cracking, thus allow concrete orother cement-based materials to be improved both directly and indirectlyin relation to durability requirements.

The use of pozzolanic materials is also increasing, and when thesematerials are very fine, such as micro-silica, they can reduce the rateof bleeding and water migration and may lead to increasing plasticcracking.

When less fine pozzolanic materials, such as pulverized fuel ash, areemployed, the rate of strength increase is reduced and the period duringwhich the concrete or other cement-based material is weak and vulnerableto plastic or early shrinkage cracking is increased. The use of groundgranulated blast furnace slag cement has a similar effect on earlystrength development. Also, when polymer emulsions are added to concreteor cement-based materials, the vulnerability to early drying cracks isknown to increase.

In all of these cases, the addition of small quantities of the very finefibres of the invention is effective in reducing the tendency of thematerial to crack, and thus enables the potential of these materials beachieved to a higher degree.

High alumina cements suffer from high exothermic temperature rises,which also lead to cracking problems and limit the effectiveness ofthese materials. The fibres of the invention are effective incontrolling such cracks and increasing the performance of these cements.

Cements which can be designed with long-term controlled expansion tooffset the long-term drying shrinkage, such as calcium sulfo-aluminates,nevertheless suffer from plastic and early drying shrinkage. The fibresof the invention are therefore also of interest in allowing thesecements to retain integrity for a sufficient time to allow the long-termshrinkage-compensating benefits of these cements to be achieved.

DETAILED DESCRIPTION OF THE INVENTION

The fibres of the invention are incorporated into a concrete or othercement-based material in the form of the above-mentioned fibre bundles,which may consist essentially of a polyolefin, a polyolefin derivative,a polyester a polyamide or a mixture of the foregoing. Typically, thefibres will consist of a polyolefin such as polypropylene orpolyethylene. Polypropylene is a well-known material for syntheticfibres, and has been used as such for many years, owing to itsresistance towards acids and bases, its advantageous strengthproperties, its low density and its low price.

While there always will be a certain variation in the number offilaments in the fibre bundles, they will typically comprise about 50 toabout 5000 filaments per bundle, such as about 100 to about 2000filaments per bundle, in particular about 500 to about 1500 filamentsper bundle, such as about 1000 filaments per bundle.

As opposed to fine fibres which for example are used in the preparationof yarn for carpets, the fibres of the invention are preferablysubstantially non-crimped, in order to facilitate dispersion in aconcrete or other cement-based material.

The individual filaments typically have a length of about 3-30 mm, e.g.about 5-25 mm, in particular about 6-18 mm, and a mean transversedimension of about 3-30 μm, such as about 5-25 μm, in particular about10-20 μm.

The aspect ratio, i.e. the ratio between length and diameter, of theindividual filaments is typically about 200-800, in particular about400-700, such as about 600. While an aspect ratio of at least about 100is considered to be the minimum desirable to achieve effectiveness fromthe use of fibres in concrete or cement-based materials, it haspreviously proved difficult to achieve good dispersion even of fibreswith lower aspect ratios. Furthermore, to disperse fibres of aspectratios of only 100 has often required special mixing facilities and theuse of special additives within the mix to assist with dispersion. Theindividual filaments according to the invention thus have a high aspectratio compared to fibres generally used in concrete, and areadvantageous as such. Nevertheless, the fibres according to the presentinvention, in the form of fibre bundles, can be readily dispersed inconcrete even when the individual filaments have an aspect ratio ofabout 1000.

The fact that the fibres of the invention are capable of being dispersedeasily in a cement mix using ordinary mixing times, procedures andequipment is believed to be due 1) the dispersibility of the unitsconstituted by the fibre bundles in a cement mix to which water has beenadded, and 2) the ready separation of the bundles into "subbundles" andindividual filaments upon mixing or agitation. The individual filamentshave a surface tension which allows them to become substantiallyhomogeneously dispersed in a concrete, mortar or paste with conventionalmixing in conventional concrete mixing equipment. The surface of thefilaments will thus be substantially hydrophilic, so that the filamentswill be easily dispersible in water or mixtures containing water, e.g. aconcrete, mortar or cement mix to which water has been added. A suitablesurface tension for the filaments is about 65-80 dynes/cm², such asabout 70-75 dynes/cm², in particular about 72-74 dynes/cm².

The desired surface tension is typically achieved by treating thebundles of filaments with a wetting agent. As an additional surfacetreatment, the bundles of fibres may optionally be subjected to anelectrical treatment known as a corona treatment. These procedures willbe explained in greater detail below.

The above-described fibres are typically produced as follows:

The first step in the production of the fibre bundles is the melting thefibre raw material(s). This often takes place in an extruder, althoughan extruder does not necessarily have to be employed. The temperatureemployed for the melting of the constituent(s) of the fibres willobviously depend on the materials employed in the given fibre.

The type of spinning equipment used in the spinning of the melt into aspun bundle of filaments is not critical, as both "short spinning" and"long spinning" may be employed. Short spinning is a one-step process,in which the bundles of fibres are both spun and stretched in a singleoperation, while long spinning, or conventional melt spinning, as italso is known, is a two-step process, in which the first step is theextrusion of the melt and the actual spinning of the bundles of fibres,while the second step is the stretching of the spun fibres.

The spun fibres are cooled as they are drawn out of the spinnerette, thecooling typically being achieved by a stream of air which is blown pastthe fibres.

The bundles of filaments, which at this point typically comprise severalthousand fibres, are subsequently stretched. Stretching is typicallyaccomplished using a series of hot rollers and a hot air oven or aliquid medium such as hot water or oil, a number of bundles of filamentstypically being stretched simultaneously. The bundles of filaments passfirst through one set of rollers, followed by passage through the hotair oven or the hot liquid, and then pass through a second set ofrollers. The hot rollers typically have a temperature of about 70°-130°C., and the hot air oven or hot liquid typically has a temperature ofabout 80°-140° C. The speed of the second set of rollers is faster thanthe speed of the first set, and the heated bundles of filaments aretherefore stretched according to the ratio between the two speeds(called the stretch ratio or draw ratio). A second oven or liquid and athird set of rollers can also be used (two-stage stretching), with thethird set of rollers having a higher speed than the second set. In thiscase the stretch ratio is the ratio between the speed of the last andthe first set of rollers. Similarly, additional sets of rollers andovens or liquids may be used.

The fibres of the present invention are typically stretched using astretch ratio of about 1.5:1-8:1, normally about 2:1-6:1, preferablyabout 2.5:1-4:1, in particular about 2.5:1-3.5:1, resulting in theappropriate diameter or mean transverse dimension as explained above.

The bundles of filaments are then dried and fixed. The stretchingprocess may cause tensions to develop in the fibres. These may berelaxed by subjecting the stretched bundles of filaments to heating,which also serves to dry the fibres. Conveniently, this is done bypassing the bundles of filaments through an oven in which the fibres areallowed to shrink.

An mentioned above, the bundles of filaments are subsequently treatedwith a wetting agent so as to provide the filaments with the desiredsurface tension, i.e. a surface tension of about 65-80 dynes/cm², suchas about 70-75 dynes/cm², in particular about 72-74 dynes/cm². This istypically accomplished by passing the bundles through a series ofso-called lubricant application rollers to which the wetting agent issupplied. In addition to providing for easy dispersion of the individualfilaments in a cement mix, the wetting agent also serves to hold thefilaments of the bundle together during light handling prior to theaddition of the fibre bundles to the mix. The wetting agent is typicallychosen from wetting agents of the kinds normally used for application tosynthetic fibres which are to be made hydrophilic, such as wettingagents for application to fibres to be used in the so-called wet-laidnon-woven processes. Such wetting agents are commercially available andare typically compositions comprising compounds normally used asemulsifiers, surfactants or detergents, and may comprise blends of thesecompounds. Examples of such compounds are fatty acid esters ofglycerides, fatty acid amides, polyglycol esters, polyethoxylatedamides, nonionic surfactants and cationic surfactants.

Specific examples of compounds which may be used as wetting agents orconstituents of wetting agents are a polyethylene glycol-lauryl etherhaving the formula:

    CH.sub.3 (CH.sub.2).sub.11 --O--(CH.sub.2 CH.sub.2 O).sub.n --H

glycerol monostearate, which has the formula:

    (C.sub.17 H.sub.35)COOCH.sub.2 CHOHCH.sub.2 OH

erucamide, which has the formula:

    C.sub.21 H.sub.41 CONH.sub.2

stearic acid amide, which has the formula:

    CH.sub.3 (CH.sub.2).sub.16 CONH.sub.2

a trialkyl-phosphate having the formula: ##STR1## alauryl-phosphate-amine ester having the formula: ##STR2## a laurylphosphate-potassium salt having the formula: ##STR3## and anethylenediamine-polyethylene glycol having the formula: ##STR4##

An example of a preferred wetting agent is SW-T which is available fromNissin Kagaku Kenkyosho Ltd., Japan, and which comprises a majorproportion of sulfosuccinic acid bis (2-ethylhexyl)ester sodium salt (ananionic wetting/dispersing compound) and also contains isopropylalcohol, siloxans, silicones, silica, and sorbitan monostearate.

The bundles of fibres may, in addition to being treated with a wettingagent, optionally be subjected to a corona treatment, which is anelectrical treatment which is widely used in the production of syntheticfibres. This treatment is a vigorous electrical discharge from a specialelectrode to the fibre bundles. A rather high voltage is required (about25 kV and 20 kHz) in order for the electrons to obtain sufficient energyto penetrate the surface of the fibres. When the electrons hit thepolymer chains at a high speed, many of these chains will be broken,thus providing the possibility of forming carbonyl groups by means ofozone (0₃) in the air. The formation of carbonyl groups makes thesurface of the fibres polar and thus more easily dispersible in aqueousmixtures. The optional corona treatment is normally performed before theapplication of the wetting agent.

After being treated with the wetting agent, the bundles of filamentsbecome spontaneously divided into smaller bundles, each of whichcomprises fewer filaments than the original bundles. Thus, the bundlesof filaments will then typically comprise about 50 to about 5000filaments per bundle, such as about 100 to about 2000 filaments perbundle, in particular about 500 to about 1500 filaments per bundle, suchas about 1000 filaments per bundle. It must be kept in mind that therewill always be a certain natural variation in the number of filamentsper bundle.

The bundles of filaments are then led to a cutter, where the fibres arecut to the desired length. Cutting is typically accomplished by passingthe bundles over a wheel containing radially placed knives. The fibresare pressed against the knives by pressure from rollers, and are thuscut to the desired length, which is equal to the distance between theknives. As explained above, the bundles of filaments are cut so that thefibres have a length of about 1-30 mm, typically about 3-30 mm, e.g.about 5-25 mm and in particular about 6-18 mm, thus providing them withan aspect ratio as explained above.

The bundles of filaments produced by the above process are, as explainedabove, designed for use in concrete, mortar or cement, and theindividual filaments of the bundles are capable of being effectivelydispersed in all types of concrete, mortar or cement using all types ofexisting conventional mixers. Accordingly, the invention also relatesto, as mentioned above, a cement-based concrete, mortar or paste havingsubstantially homogeneously distributed therein the above-describedsynthetic fibres, the fibres being present in an amount of less thanabout 1% by weight of the cementitious materials of the concrete, mortaror paste.

In the present context, the term "cement" is intended to designate allcements of the Portland cement type, including white Portland cement,low-alkali cements, sulphate-resistant cements, Portland slag cement andPortland pozzolana cement, and cements of the refractory or aluminatetype such as high alumina cement and calcium sulfoaluminate cements,blast furnace cements, pozzolanic cements, gypsum including hemi andanhydrite versions, magnesium oxychlorate and magnesium chloride andother similar non-organic cement systems, both hydraulic andnon-hydraulic, or combinations of the above, optionally with additivesor polymer additions. A "paste" refers to a mixture of cement and water.

The term "mortar" as used in the present context refers to a mixturecomprising cement and particles such as sand and fine rock or stone,including special lightweight aggregate materials, the particles beingable to pass through a sieve or screen having an opening of 2.4 mm. Theterm "concrete" as used in the following refers to a mortar or pastecomprising larger aggregates. The term "cementitious materials" refersto the content of the above-mentioned cement materials in a concrete,mortar or paste.

It will be clear to a person skilled in the art that the term"substantially homogeneously distributed therein" refers to the factthat the fibres of the invention are substantially homogeneouslydistributed within the mortar phase of the material of the invention,since such fibres clearly cannot be distributed within the largeraggregates in a concrete.

Concrete or other cement-based materials can be regarded as being eitherin situ or pre-cast, in situ materials being cast on-site. In situconcrete is generally of the ready-mixed type, although it may also bemixed on-site.

A mortar or concrete comprising the fibres of the invention willgenerally have a cement content in the range of about 200-1200 kg/m³. Aconventional in situ concrete, in which the fibres of the inventionoften will be incorporated, typically has a cement content of about200-600 kg/m³, in particular about 250-450 kg/m³, while a pre-castconcrete employing the fibres of the invention will typically have acement content of about 300-500 kg/m³. A mortar will typically have acement content of about 400-1200 kg/m³, in particular about 600-1000kg/m³. Special high-strength concretes or mortars may have a cementcontent of about 500-1200 kg/m³, typically about 500-1000 kg/m³.

The water:cement ratio of a cement-based material according to theinvention will typically be in the range of about 0.25-0.8 by weight. Insitu concrete will typically have a water:cement ratio of about 0.4-0.6,while the water:cement ratio in pre-cast concrete will typically beabout 0.25-0.35 when compacted by pressure and about 0.4-0.6 when wetcast and vibration compacted. However, the incorporation of the fibresof the invention into cement-based materials having a water:cement ratiolower than 0.25, e.g. dense materials containing ultrafine micro-silica,is also of interest.

A concrete according to the invention will typically contain a mortarphase of at least about 0.2 by weight. The proportion of the mortarphase in a conventional concrete is generally limited to a maximum ofabout 0.6, due to the fact that the concrete's tendency to crackincreases with increasing amounts of mortar. However, since theincorporation of the fibres of the invention into concrete leads to areduced tendency to cracking, it is possible to produce concretes with alarger mortar phase than that which is normally employed, without thedanger of excessive cracking. Thus, a concrete according to theinvention may comprise a relatively large mortar phase, such as up toabout 0.8 or even greater.

The fibres of the invention are typically present in the material in anamount of about 0.05-0.5%, in particular about 0.1-0.3%, such as0.15-0.25%, by weight of the cementitious materials.

For a typical in situ concrete having a cement content of about 250-400kg/m³, the content of fibres of the invention will thus be less thanabout 4 kg/m³, typically about 0.1-2.0 kg/m³, such as about 0.3-1.0kg/m³, in particular about 0.4-0.8 kg/m³, e.g. about 0.5-0.7 kg/m³.

A concrete or mortar according to the invention may contain additives toreduce water requirement, increase workability, change rheology, reducepermeability, entrain air or retard or accelerate the cement reactionwith water. It may also contain various types of organic polymersintroduced as solids or water-based emulsions, includingpolymer-impregnated concrete or polymer cement concrete. In addition, itmay contain reinforcement, included either as bars or meshes, includingferrous cement and metal lathing, or as additional fibres of e.g. metal,glass or synthetic material.

As explained above, it has been found that the incorporation of evenvery small quantities of the fibres of the invention provides thecement-based material in question with various advantages. That suchbenefits can be obtained with such small quantities of fibres can beexplained by the fineness of the fibres, together with the fact thatthey are able to become substantially homogeneously dispersed in thematerial. Fibre bundles of the invention having, for example, 300×10⁶individual 12 mm long filaments per kg will provide, when incorporatedinto a cement-based material at a rate of, for example, 0.6 kg of fibrebundles per m³, about 2000 km of fibre per m³. Seen in this light, it isclear that even small quantities of the fibres of the invention canprovide significant benefits when incorporated into a cement-basedmaterial.

The cement-based material of the invention can, as explained above, beproduced by adding to a concrete, mortar or cement mix to which waterhas been added less than 1% by weight, based on the cementitiousmaterials, of the fibre bundles according to the invention, mixing theresulting mix for a period of at least about 20 seconds to obtain aconcrete, mortar or paste mix in which the individual filaments aresubstantially homogeneously distributed, and casting the concrete,mortar or paste mix in a desired configuration, optionally withincorporation, during the casting, of additional bodies such asreinforcement. The fibre bundles are typically added in an amount ofabout 0.05-0.5%, in particular about 0.1-0.3%, such as about 0.15-0.25%,by weight of the cementitious materials.

As the individual filaments of the fibre bundles are readily dispersedin all types of concrete and cement-based materials, the mixing periodis dictated by the need to produce good concrete, rather than todisperse the fibres. The bundles of fibres according to the inventioncan be used in all forms of mixers, including rotating drum and paddlemixers and in particular ready-mixed concrete truck mixers, and requireno special mixing arrangements or equipment. In cases where a pre-castconcrete, mortar or cement mix to which the fibre bundles have beenadded are mixed in a paddle mixer (also known as a forced action mixer),mixing is carried out for a period of at least about 20 seconds,typically at least about 30 seconds, to obtain a concrete, mortar orpaste mix in which the individual filaments are substantiallyhomogeneously distributed. In cases where an in situ concrete, mortar orcement mix to which the fibre bundles have been added are mixed in adrum mixer (also known as a tumble mixer), mixing is typically carriedout for a period of at least about 2 minutes, to obtain a concrete,mortar or paste mix in which the individual filaments are substantiallyhomogeneously distributed.

The bundles of fibres of the invention will often be added to a concretemix in a truck mixer, the truck mixer being an arrangement consisting ofa spiral inside of an inclined drum. When the drum rotates, the materialbeing mixed simply falls to the bottom of the spiral, and thatconstitutes the mixing action. The fibre bundles can also be added toalready mixed concrete, and good dispersion can be achieved with theready-mix drum rotating at, for example, 15 rpm for a period of, forexample, 3 minutes.

A truck mixer can be designed either for mixing or for agitation only.In some systems concrete is mixed and put in the drum of a truck mixer,so that the truck is merely used to agitate the already mixed concrete,while in other systems the materials of the concrete are put in the drumof the truck mixer, and the truck mixer actually mixes the materials.

It is possible to mix the fibre bundles into the dry constituents of acement or concrete mix, e.g. in pre-blended dry mixed materialsrequiring only the addition of water, but at present, this is neitherpreferred nor considered necessary, as it is believed to be at least asadvantageous to add the fibre bundles to a wet mixture or a mixture towhich water already has been added, due to the substantially hydrophilicsurface properties of the fibres.

The mixed concrete, mortar or paste comprising the fibres of theinvention substantially homogeneously dispersed therein can be cast in aconventional manner in a desired configuration. The material may thus becompacted and shaped either by simple placing or gravity, or bytrowelling, floating, tamping, vibration, pressing, water extraction,vacuum, extrusion, pumping, spraying, dry placing, spinning, rolling ora combination of these processes. Additional bodies, such asreinforcement, can, if desired or necessary, be incorporated into thematerial during casting.

Materials prepared according to the invention are envisaged as being ofparticular importance in all types of mass on-site concrete, such as forpavements, foundations, roadways, floors, bridge decks, concretebuildings, structural concrete, retaining walls, water retainingstructures and for sea defence and military purposes, as well as inpre-cast concrete, such as for cladding panels, floors, joists andbeams, ornamental and architectural products, prefabricated structures,pipes, tunnel linings, etc.

The invention will be further illustrated by the following non limitingexamples.

EXAMPLE 1

Preparation of fibre bundles

The preparation of the fibre bundles comprised the following steps:

melting the fibre raw material to obtain a melt,

spinning the melt into a spun bundle of filaments,

stretching the bundle of filaments,

drying and fixing the bundle of filaments,

treating the bundle of filaments with a wetting agent, and

cutting the bundles of filaments.

The fibres consisted of a homopolymer isotactic polypropylene (Petrofina10060 from Petrofina, Belgium) having a melting point of about 160° C.and a melt flow index of 35. The polypropylene was melted, andsubsequently spun at a temperature of about 280° C., using a spinnerettehaving 22,880 holes, and with a drawing speed of 22.5 m/min. The spunbundle of filaments was then passed through a hot water bath having atemperature of 100° C. and subsequently stretched at a speed of 60.7m/min., giving a stretch ratio of 2.7. Drying and fixing of the bundleof filaments was accomplished by passing the bundle through an ovenhaving a temperature of 150° C. at a speed of 54.2 m/min., thus allowingthe fibres to shrink by about 12% and the tensions from the stretchingof the fibres to be released. The fibres were provided with the desiredsurface tension by treating the bundle with a wetting agent (SW-T,Nissin Kagaku Kenkyosho Ltd., Japan, vide above) by passage through apair of lick rollers. Finally, the bundles of fibres were cut to alength of 12 mm.

The finished fibres, comprising roughly 1000 individual filaments perbundle, had a moisture content of less than 17% and contained about 0.5%wetting agent. The individual filaments had a fineness of 2.8 dtex,which is the equivalent of a diameter of about 20 μm, giving the fibresan aspect ratio of about 600.

EXAMPLE 2

Preparation of concrete beams

Concrete beams were prepared from a factory pre-blended concrete mixconsisting of rapid-hardening Portland cement, standard coarseconcreting sand and a gravel aggregate passing a 20 mm sieve, in a ratioof 2:3:6, the mix having a cement content of 400 kg cement/m³. Theconcrete mix was mixed with water having a temperature of 20° C., aswell as the fibre bundles of Example 1. The water:cement ratio was 0.6,and the fibre bundles were added in an amount of 0.2% by weight of thecementitious materials. The concrete was mixed in a rotating drum mixerwith a capacity of about 100 l, using a speed of about 25 rpm and atotal mixing time of 4 minutes, the fibre bundles being added after thefirst 2 minutes of mixing. The individual filaments were substantiallyhomogeneously distributed in the mix at the end of the mixing period.Concrete beams of 150 mm square and 550 mm long were prepared by placingthe mix into a multi-compartment mold and compacting by hand.

For purposes of comparison, beams were prepared as above, but withoutthe incorporation of the fibres. Beams with and without fibres were castalternately in the multi-compartment mold.

EXAMPLE 3

Bending tests

Laboratory tests were carried out on concrete beams prepared as inExample 2, containing either 0.2% fibres by weight of the cementitiousmaterials or without fibres. The beams were subjected to early drying,and subsequently subjected to a standard 4-point flexural bending test,with outer rollers spaced at 450 mm and inner rollers spaced at 150 mm.

The results of these tests are summarized below:

    ______________________________________                                        Fibres        Modulus of rupture                                              ______________________________________                                        without fibres                                                                              2.83 MPa                                                        without fibres                                                                              2.31 MPa                                                        0.2% fibres   3.17 MPa                                                        0.2% fibres   3.22 MPa                                                        ______________________________________                                    

It is seen that the modulus of rupture is considerably higher for thebeams which contain the fibres of Example 1. The modulus of rupture wasalso more consistent in beams containing the fibres.

EXAMPLE 4

Cyclical loading tests

Concrete beams were prepared as in Example 2 and subjected to a 4-pointflexural bending test as in Example 3, with the following exceptions: 1)the beams were not subjected to early drying, and 2) the load was keptbelow the modulus of rupture and was repeatedly applied at 2000cycles/hour. The load was increased after 4000 cycles in order to reducethe period of the test. The results are summarized below:

    ______________________________________                                        Maximum                                                                       modulus      No. of cycles at load of                                         Fibre  of rupture                                                                              15 kn   16 kn 17 kn 18 kn 19 kn                              ______________________________________                                        without                                                                              2.13 MPa  1540     425                                                 without                                                                              2.27 MPa          4160   17                                            with   2.40 MPa          4000  4000   925                                     with   2.53 MPa          4000  4000  4000  1875                               ______________________________________                                    

The tests indicated a distinct improvement in both stress levels andfatigue resistance in beams containing the fibres. The combination ofboth the increased stress levels and the numbers of cycles beforefailure with the fibre-containing concrete indicates considerablyimproved fatigue resistance.

EXAMPLE 5

Preparation of in situ concrete containing the fibres of the invention

The fibre bundles of Example 1 have been incorporated in an amount of0.2% by weight of the cementitious materials into various types ofconcrete, including in situ concrete, using conventional unmodifiedmixing equipment, and without the need for additional additives, asfollows:

a) A 30 MPa concrete, with 300 kg of cement per cubic meter and a 20 mmaggregate, with a water cement ratio of 0.56 and a 50 mm slump.

b) A 30 MPa concrete, as in a) above but with 320 kg of cement per cubicmetre and a water;cement ratio of 0.52, and including an air-entrainingagent.

c) A 30 MPa concrete, as in a) above but with 210 kg of ordinaryPortland cement and 105 kg of pulverized fuel ash per cubic metre.

d) A 30 MPa concrete, with 350 kg of cement per cubic metre and roughlyequal proportions of a 10 mm aggregate and sand, with a water:cementratio of 0.58 and a 100 mm slump.

e) A 40 MPa concrete, with 400 kg of cement per cubic metre and a 10 mmaggregate, with a water:cement ratio of 0.50 and a 100 mm slump.

All of the above concretes were mixed in conventional truck mixersmanufactured by Mulder and Stothert & Pitt and containing 6 m³ concrete.In all cases complete dispersion of the fibres was achieved within 3minutes with the drum rotating at 15 rpm. This complete dispersion wasachieved even when the fibres were added to the already mixed concreteby simply introducing the fibres into the back of the truck mixer onsite.

EXAMPLE 6

Preparation of pre-cast concrete containing the fibres of the invention

The following pre-cast concrete materials containing the fibres of theinvention, incorporated as the fibre bundles of Example 1, in an amountof 0.2% by weight of the cementitious materials, were prepared:

a) A 40 MPa concrete containing 400 kg of cement per cubic metre androughly equal proportions of 5 mm gravel and sand, with a water:cementratio of 0.31.

b) A 40 MPa concrete containing 350 kg of cement per cubic metre androughly equal proportions of 10 mm gravel and sand, with a water:cementratio of 0.30.

Full dispersion of the fibres was achieved in both cases in under 1minute in a forced action paddle mixer (Teka and Liner Cumflow). Theconcrete compacted well in both cases, and the products exhibited noadverse surface effects.

EXAMPLE 7

Concrete pavement containing the fibre of the invention

A 30 MPa concrete containing about 300 kg of cement per cubic meter anda 20 mm aggregate, with a water:cement ratio of 0.55, and containing0.2%, by weight of the cementitious materials, of fibres according tothe invention incorporated as the fibre bundles of Example 1, wasprepared. 200 mm thick in situ concrete pavement areas were placedexternally in strips 2.5 m wide and in continuous lengths of 50 m, withno shrinkage control joints. No cracking was evident after 2 months, andthus it is not anticipated that cracking will occur.

Similar trials, which gave similar results, had been undertaken 9 monthsearlier with continuous strips having a length of 20 m.

EXAMPLE 8

Practical experience with concrete prepared with the fibres of theinvention

a) A 200 mm thick concrete roadway with a width of approximately 5meters and a length of just over 80 meters with a gradient of about 1 in15 was laid using 0.7 kg of the fibres of Example 1 pr. m³ concrete. Theconcrete was 30 MPa, air entrained, with a total cementitious content of330 kg/m³ including 25% of furnace blast slag.

The 80 meters was laid in one day in a continuous strip formed from thetop of the incline to the base. No shrinkage control Joints were formed.The top of the concrete was thickened so that it was effectivelyanchored and the end of the roadway was stopped short of the adjoiningconcrete at the base, this being filled in later.

After some weeks a single fracture occurred across the road atapproximately the mid-point and in line with a joint in adjoiningconcrete. After being in use for nearly a year by a continuous stream ofheavy lorries, dump trucks, etc., no deterioration or further crackinghas appeared. The central crack has not opened, nor has any differentialvertical movement on either side of the crack occurred, the crack infact being visible only by careful examination.

This and other applications indicate that the preparation of continuousstrips of up to about 50 linear meters without shrinkage control jointsseems perfectly feasible with the fibres of the invention and withoutany steel reinforcement.

b) The fibres of the invention were used in a factory floor of laserscreed concrete.

The concrete was laid in two pours, each in one day, the first pourbeing of 2300 m² and the second pour of 3200 m², the concrete being astandard 30 MPa concrete with 330 kg/m³ of ordinary Portland cement. Theconcrete contained the fibres of Example 1 in an amount of 0.6 kg/m³.The surface was power floated and was treated on the following day withsodium silicate as a surface hardener. The concrete was laid at a 150 mmthickness onto a polyethylene vapour barrier.

Two days after laying, the concrete was sawn at stanchion spacing orroughly every 7 meters to a depth of approximately 50 mm, to formlong-term shrinkage control joints.

After several months, a number of the sawn control joints had opened,but no cracking of the concrete had otherwise taken place.

Because of the very cold weather following the laying of the concrete,the factory radiant gas heaters were left on for 48 hours, but despitethis, no shrinkage cracks occurred.

c) Over 4 tons of the fibres of the invention have been used in seadefence work to protect large areas of low lying land in Lincolnshire,England from being flooded by the North Sea. Two grades of concrete havebeen used, a 30 MPa and a 40 MPa, both of which had high cementsubstitutions to control the alkali content.

Serious plastic cracking has always been experienced in sea defenceconcrete because of the very windy and exposed conditions. It was foundthat the addition of 0.9 kg of the fibres of Example 1 led to adramatically reduced incidence of cracking, and the overall resultsobtained have proved very satisfactory.

d) Approximately 100,000 m² of ground floor car park concrete containingthe fibres of the invention was laid at a shopping development.

150 mm thick concrete slabs were prepared with incorporation of bottomsteel mesh, but without incorporation of top steel mesh, the concreteinstead being prepared with 0.9 kg of the fibre bundles of Example 1 pr.cubic metre. The concrete was a 30 MPa concrete, 330 kg/m², airentrained with a 25% slag cement replacement.

The preparation of such large concrete slabs, which had a size of 8×16meters, would normally be extremely difficult, since the use of thebottom steel mesh restrains overall slab shrinkage and consequentlyinduces greater shrinkage stresses in the concrete. However, no crackingwas exhibited in the concrete slabs containing the fibres of theinvention, and the overall results obtained have been excellent.

e) A factory floor of over 6000 m² with a general thickness of 150 mmwas laid using conventional strip laying with sawn shrinkage controljoints.

The concrete was a high-strength concrete containing micro-silica andwith a cement content of 350 kg/m², and was prepared using a standardsuperplasticizer to reduce the water/cement ratio to below 0.5. Thefibres of Example 1 were used in an amount of 0.6 kg/m³.

It was found that the cube strengths actually achieved were consistentlyhigher than anticipated. The results have proved excellent and indicatethat the combination of high-strength concrete, which is generallyrecognized as being more prone to cracking, with the crack-inhibitingeffect of the fibres is an excellent combination.

We claim:
 1. A method for producing a cement material with reduceddevelopment of self-induced cracking, the method comprising:adding to aconcrete, mortar or cement mix to which water has been added, an amountof 0.05 to 1% by weight, based on the cementitious materials, ofsynthetic fibre bundles comprising 10-10,000 filaments per bundle, thefilaments consisting essentially of a polyolefin, a polyolefinderivative, a polyester or a mixture of the foregoing and having alength of 1 to 30 mm, a mean transverse dimension of 5 to 30 μm and anaspect ratio of 100 to 1000, the filaments in each bundle being heldtogether by a wetting agent, the wetting agent providing the individualfilaments with a surface tension which allows them to becomehomogeneously dispersed in a concrete, mortar or paste with conventionalmixing in conventional concrete mixing equipment, mixing the resultingmix for a period of at least about 20 seconds to obtain a concrete,mortar or paste mix in which the individual filaments are homogeneouslydistributed, and casting the concrete, mortar or paste mix in aconfiguration.
 2. The method of claim 1 wherein the wetting agent isselected from the group consisting of fatty acid esters of glycerides,fatty acid amides, polyglycol esters, polyethoxylated amides, nonionicsurfactants, cationic surfactants and blends of the above.
 3. The methodof claim 1 wherein a pre-cast concrete, mortar or cement mix to whichthe fibre bundles have been added are mixed in a paddle mixer for aperiod of at least about 30 seconds to obtain a concrete, mortar orpaste mix in which the individual filaments are homogeneouslydistributed.
 4. The method of claim 1 wherein an in situ concrete,mortar or cement mix to which the fibre bundles have been added is mixedin a drum mixer for a period of at least about 2 minutes to obtain aconcrete, mortar or paste mix in which the individual filaments arehomogeneously distributed.
 5. The method of claim 1 wherein thesynthetic fibre bundles are added in an amount of 0.05-0.5% by weight ofthe cementitious materials.
 6. The method of claim 5 wherein thesynthetic fibre bundles are added in an amount of 0.1-0.3% by weight ofthe cementitious materials.
 7. The method of claim 6 wherein thesynthetic fibre bundles are added in an amount of 0.15-0.25% by weightof the cementitious materials.
 8. The method of claim 1 wherein thefibres are non-crimped.
 9. The method of claim 1 wherein the individualfilaments have a length of 3-30 mm.
 10. The method of claim 9 whereinthe individual filaments have a length of 6-18 mm.
 11. The method ofclaim 1 wherein the individual filaments have a mean transversedimension of 5-25 μm.
 12. The method of claim 11 wherein the individualfilaments have a mean transverse dimension of 10-20 μm.
 13. The methodof claim 1 wherein the fibres consist essentially of polypropylene. 14.The method of claim 1 wherein the filaments have an aspect ratio of200-800.
 15. The method of claim 14 wherein the filaments have an aspectratio of 400-700.
 16. Synthetic fibre bundles for the prevention of theformation of self-induced cracks in a cement material comprising10-10,000 filaments per bundle, the filaments consisting essentially ofa polyolefin, a polyolefin derivative, a polyester or a mixture of theforegoing and having a length of 1 to 30 mm, a mean transverse dimensionof 5 to 30 μm and an aspect ratio of 100 to 1000, the filaments in eachbundle being held together by a wetting agent, the wetting agentproviding the individual filaments with a surface tension which allowsthem to become homogeneously dispersed in a concrete, mortar or pastewith conventional mixing in conventional concrete mixing equipment. 17.Synthetic fibre bundles according to claim 16 wherein the wetting agentis selected from the group consisting of fatty acid esters ofglycerides, fatty acid amides, polyglycol esters, polyethoxylatedamides, nonionic surfactants, cationic surfactants and blends of theabove.
 18. Synthetic fibre bundles according to claim 16 wherein thefibres are non-crimped.
 19. Synthetic fibre bundles according to claim16 wherein the individual filaments have a length of 3-30 mm. 20.Synthetic fibre bundles according to claim 19 wherein the individualfilaments have a length of 5-25 mm.
 21. Synthetic fibre bundlesaccording to claim 20 wherein the individual filaments have a length of6-18 mm.
 22. Synthetic fibre bundles according to claim 16 wherein theindividual filaments have a mean transverse dimension of 5-25 μm. 23.Synthetic fibre bundles according to claim 22 wherein the individualfilaments have a mean transverse dimension of 10-20 μm.
 24. Syntheticfibre bundles according to claim 16 wherein the filaments have an aspectratio of 200-800.
 25. Synthetic fibre bundles according to claim 24wherein the filaments have an aspect ratio of 400-700.
 26. Syntheticfibre bundles according to claim 16 which consist essentially ofpolypropylene.
 27. A method for the prevention of the development ofself-induced cracking in a cement material, the method comprising addingto a concrete, mortar or cement mix to which water has been added, anamount of 0.05 to 1% by weight, based on the cementitious materials, ofsynthetic fibre bundles comprising 10-10,000 filaments per bundle, thefilaments consisting essentially of a polyolefin, a polyolefinderivative, a polyester or a mixture of the foregoing and having alength of 1 to 30 mm, a mean transverse dimension of 5 to 30 μm and anaspect ratio of 100 to 1000, the filaments in each bundle being heldtogether by a wetting agent, the wetting agent providing the individualfilaments with a surface tension which allows them to becomehomogeneously dispersed in a concrete, mortar or paste with conventionalmixing in conventional concrete mixing equipment.
 28. A concrete, mortaror paste produced according to the method of claim
 1. 29. A concrete,mortar or paste produced according to the method of claim
 2. 30. Aconcrete, mortar or paste produced according to the method of claim 3.31. A concrete, mortar or paste produced according to the method ofclaim
 4. 32. A concrete, mortar or paste produced according to themethod of claim
 5. 33. A concrete, mortar or paste produced according tothe method of claim
 6. 34. A concrete, mortar or paste producedaccording to the method of claim
 7. 35. A concrete, mortar or pasteproduced according to the method of claim
 8. 36. A concrete, mortar orpaste produced according to the method of claim
 9. 37. A concrete,mortar or paste produced according to the method of claim
 9. 38. Aconcrete, mortar or paste produced according to the method of claim 10.39. A concrete, mortar or paste produced according to the method ofclaim
 11. 40. A concrete, mortar or paste produced according to themethod of claim
 12. 41. A concrete, mortar or paste produced accordingto the method of claim
 13. 42. A concrete, mortar or paste producedaccording to the method of claim
 14. 43. A concrete, mortar or pasteproduced according to the method of claim 15.